Medium inspection plays an important role in medical imaging, non-destructive evaluation and remote sensing. Ultrasonic based systems and X-ray based systems are widely used for material inspection. The inspection methods based on ultrasound use high frequency sound waves to detect and image features of a material. Pulsed beams of ultrasound can be produced with a transducer. Any sound that returns to the transducer gives two main measurements: amplitude of returned signal and time elapsed between emission and reception by the transducer. A common limitation of sonic systems is the poor resolution, which is caused by the low operation frequencies. X-ray system is a precise and efficient way to inspect the material. However, there are several limitations in this technology: i) the price of the X-ray system is expensive; and ii) X-ray exposure of the operators must be limited.
Similar to X-ray, microwave can “see through” the target. The technique utilizing the property has being developed very fast recently. This technique is efficient, can provide high resolution images for the target medium by making use of the reflection, scattering, diffraction and attenuation properties of electromagnetic fields. In addition, the generation and control of microwave are easier compared with X-ray, ultrasonic and other imaging technologies. Furthermore, with the development of wireless communications, the components required in the microwave system are inexpensive.